vertical parallax image scroller

ObservableScrollView.java

package me.schilter.faactors.view;

import android.content.Context;
import android.util.AttributeSet;
import android.widget.ScrollView;

/**
 * ScrollView with a callback
 *
 * @author Roland Schilter <roli@schilter.me>
 */
public class ObservableScrollView extends ScrollView {

    private ObservableScrollViewListener mScrollViewListener = null;

    public interface ObservableScrollViewListener {
        void onScrollChanged(ObservableScrollView scrollView, int x, int y, int oldx, int oldy);
    }

    public ObservableScrollView(Context context) {
        super(context);
    }

    public ObservableScrollView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    public ObservableScrollView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public void setScrollViewListener(ObservableScrollViewListener scrollViewListener) {
        mScrollViewListener = scrollViewListener;
    }

    @Override
    protected void onScrollChanged(int x, int y, int oldx, int oldy) {
        super.onScrollChanged(x, y, oldx, oldy);
        if (mScrollViewListener != null) {
            mScrollViewListener.onScrollChanged(this, x, y, oldx, oldy);
        }
    }
}

ParallaxImageView.java

package me.schilter.faactors.view;

import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.widget.ImageView;

/**
 * Image view that shifts the image vertically while scrolling
 *
 * Make sure to set the translation in your OnScrollListener
 *
 * @author Roland Schilter <roli@schilter.me>
 */
public class ParallaxImageView extends ImageView {
    protected int mTranslation = 0;

    public ParallaxImageView(final Context context)
    {
        super(context);
    }

    public ParallaxImageView(final Context context, final AttributeSet attrs)
    {
        super(context, attrs);
    }

    public ParallaxImageView(final Context context, final AttributeSet attrs, final int defStyle)
    {
        super(context, attrs, defStyle);
    }

    public void setTranslation(int translation) {
        mTranslation = translation;
        invalidate();
    }

    @Override
    public void draw(Canvas canvas) {
        canvas.save();
        canvas.translate(0, mTranslation/2);
        super.draw(canvas);
        canvas.restore();
    }
}

SomeFragment.java

[...]

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = (LinearLayout) inflater.inflate(R.layout.movie_detail_fragment, container, false);

        ObservableScrollView scroller = (ObservableScrollView) view.findViewById(R.id.scrollview);
        final ParallaxImageView backdrop = (ParallaxImageView) scroller.findViewById(R.id.backdrop_picture);

        scroller.setScrollViewListener(new ObservableScrollView.ObservableScrollViewListener() {
            @Override
            public void onScrollChanged(ObservableScrollView scrollView, int x, int y, int oldx, int oldy) {
                backdrop.setTranslation(y);
            }
        });

        return view;
    }

[...]